1. Field of the Invention
The present invention a driving circuit for a liquid crystal device. More particularly, the present invention relates to a liquid crystal driving circuit for driving a liquid crystal display screen in a personal digital assistant, etc.
2. Description of the Related Art
As a display means of a personal digital assistant such as a pager, a cellular phone, an electronic pocketbook, etc., a low power consumption liquid crystal element is employed. As a liquid crystal element driving system, there is a low power consumption driving system which employs a voltage step up/down circuit using capacitors and which is mainly employed in low duty display such as numerals, alphabets, etc. In contrast, there is employed a driving system which employs an operational amplifier and which is employed in high duty display such as Chinese characters, characters, etc. In this system, a large power is consumed because a large current flows through the operational amplifier. Today a larger display screen of the liquid crystal display, i.e., higher duty of the liquid crystal has been advanced with the progress of multi-function of the personal digital assistant. It is certain that such high duty display will become the mainstream of the liquid crystal display in the near future. Therefore, the low power consumption liquid crystal driving circuit is also earnestly desired in the field of the high duty display.
A liquid crystal driving circuit to enable the high duty display in the prior art will be explained hereunder. FIG. 1 is a circuit diagram showing a configuration of the liquid crystal driving circuit employed for the high duty display in the prior art. In this liquid crystal driving circuit, voltage dividing resistors 103 to 105, one of bias selection resistors 106 to 109, and voltage dividing resistors 110 and 111 are connected in series between the supply voltage Vdd 101 for generating the liquid power supply and the reference voltage Vss 102. Thus, intermediate potentials can be generated according to respective resistance values of the bias selection resistors 106 to 109. The voltage dividing resistor 103 is a liquid crystal temperature compensating resistor whose resistance value RA can be varied by the software control.
In general, a proper value of a liquid crystal bias voltage VC1 in the liquid crystal using the TN (Twisted Nematic) method or the STN (Super Twisted Nematic) method can be given by EQU VC1=1/((duty).sup.1/2 -1) to 1/((duty).sup.1/2)+1 (1)
This liquid crystal bias voltage VC1 can be decided by selecting any one of the bias selection resistors 106 to 109. This selection of the bias selection resistors 106 to 109 is made by decoding 2-bit signals R1, R2 by using a decoder 112 in the publicly known technology and then turning ON any one of analogue switches 113 to 116 selectively based on an output signal of the decoder 112.
Normally the voltage dividing resistors 104, 105, 110, 111 are set to have the same resistance value and the resistance values of the bias selection resistors 106 to 109 are set N times larger than that of the voltage dividing resistors 104, 105, 110, 111. Usually, 2 to 5 is used as the value N. For example, in case the resistance value of the voltage dividing resistors 104, 105, 110, 111 is assumed as RB, the resistance value of the bias selection resistor 109 is selected as 2RB, the resistance value of the bias selection resistor 108 is selected as 3RB, the resistance value of the bias selection resistor 107 is selected as 4RB, and the resistance value of the bias selection resistor 106 is selected as 5RB. Accordingly, the liquid crystal bias voltage VC1 becomes 1/6 bias if the bias selection resistor 109 is selected, the liquid crystal bias voltage VC1 becomes 1/7 bias if the bias selection resistor 108 is selected, the liquid crystal bias voltage VC1 becomes 1/8 bias if the bias selection resistor 107 is selected, and the liquid crystal bias voltage VC1 becomes 1/9 bias if the bias selection resistor 106 is selected.
In this liquid crystal driving circuit, the resistors 103 to 111 are set to have high resistance such that the direct current flowing through them should be suppressed as small as possible. The intermediate potentials generated by using the resistors 103 to 111 are amplified by operational amplifiers 117 to 121. As a result, sufficient current to drive the large size liquid crystal display screen can be generated. Thus, outputs of the operational amplifiers 117 to 121 are stored in the capacitors 122 to 126 to be stabilized.
FIG. 2 is a view showing behaviors of driving waveforms of a common bias voltage COM and a segment bias voltage SEG when the analogue switch 116 in FIG. 1 is turned ON to select the resistor 109 and thus to set the liquid crystal bias voltage VC1 to 1/6 bias. In FIG. 2, the liquid crystal element is brought into its energized state only in a period of time when potential difference between the segment bias voltage SEG and the common bias voltage COM is within .+-.VLC, and it is brought into its non-energized state in other periods of time. As shown in FIG. 3, the COM-based SEG becomes .+-.VLC in the energized state and becomes VLC3-VLC4 (=+VLC/6) or VLC2-VLC1 (=-VLC/6) in the non-energized state.
However, in the liquid crystal driving circuit in the prior art shown in FIG. 1, the direct current always flows through the resistors 103 to 111 and also the large current is consumed in the operational amplifiers 117 to 121 which are employed to amplify the generated intermediate potential. Since these currents always flow during the display operation, such currents have caused a serious problem to achieve lower power consumption of the personal digital assistant, etc.